1. Field of the Invention
The invention relates generally to the field of measurement while drilling (“MWD”) instrumentation. More particularly, the invention relates to structures for providing wellbore hydrostatic pressure compensation and fluid sealing for rotating shafts in a wellbore instrument coupled to a drill string.
2. Background Art
MWD instruments are used for, among other purposes, measuring the trajectory of wellbores drilled through the Earth's subsurface. A typical MWD instrument is configured to be coupled in the lower portion of a drill string used to drill the subsurface formations, and includes geodetic trajectory sensing devices, called “directional sensors” that measure one or more parameters related to the geodetic orientation of the MWD instrument. Geodetic orientation of the MWD instrument can be used to determine geodetic trajectory of the wellbore at the longitudinal position of the MWD instrument. Typical MWD instruments also include one or more forms of signal telemetry so that the measurements made by the directional sensors can be transmitted to control units at the Earth's surface. The measurements may be used at the surface to enable the wellbore operator to change the trajectory as desired.
One type of telemetry known in the art is referred to as a “mud siren” and which includes a rotating shaft driven by a motor in the instrument. The shaft rotates a rotor having a selected pattern of one or more flow orifices therein. The rotor is disposed proximate a stator, which itself includes one or more orifices or features that cooperate with the orifice(s) on the rotor. The rotor and stator are disposed inside the drill string so as to affect the flow of drilling fluid through the drill string in a certain manner. By suitable rotation of the motor, and thus the shaft and rotor, flow of drilling mud through the interior of the drill string can be modulated to communicate the signals from the directional sensor to the Earth's surface. Such telemetry is referred to as “mud pulse” telemetry.
It is necessary for operation of the shaft for at least part of the shaft to be enclosed in a substantially sealed chamber. The chamber is typically filled with bit or other electrically non-conductive, lubricating and particle free liquid to as to protect bearings that rotatably support the shaft from intrusion of drilling mud. It is also necessary to provide a seal around the shaft that enables rotation thereof while excluding mud from bypassing the seal. A typical seal element is called a “face seal” and consists of a planar surface coupled to the shaft and a corresponding surface coupled to the housing that supports the shaft placed proximate each other. The surfaces are typically ceramic, tungsten carbide or similar wear resistant material. A reservoir of fluid (typically oil) is disposed in the MWD instrument and is maintained at a selected pressure referenced to the external hydrostatic pressure of the drilling mud. It is preferable that the reservoir pressure is maintained at least as high as, and preferably slightly higher than the external hydrostatic pressure such that a small leakage is created across the shaft seal. Such leakage may clean the seal, lubricate the seal and prevent accumulation of particulate matter from the drilling mud from accumulating on the seal surfaces, thus reducing the chance of seal damage. In some MWD instruments known in the art, the length of the shaft results in the shaft having significant flexibility. Therefore, in such instruments, the seal is typically articulated using an elastomer ring so that any bending of the shaft does not result in excessive clearances between the seal surfaces.
In order to maintain the appropriate pressure in the oil reservoir as the instrument traverses the wellbore and is exposed to a wide range of external hydrostatic pressure in the drilling mud, which increases linearly with vertical depth of the wellbore, typically MWD instruments include a pressure compensator that causes the reservoir to be exposed to mud pressure in the drill string while excluding mud from entering the reservoir. Pressure compensators are typically either an elastomer bladder filled with oil, externally subjected to drill string fluid and internally coupled to the reservoir (or forming the reservoir) or a piston that is exposed to drill string fluid in one side and is in hydraulic communication with the reservoir on the other side. A suitable hydrostatic pressure reference position is carefully selected for the pressure compensator because there is significant fluid pressure drop through the MWD instrument. If a pressure reference is selected that is subject to substantial pressure drop, the reservoir pressure may be inadequate for proper seal operation and may result in mud intrusion into the reservoir and hydraulic system. Further, inadequate pressure compensation may enable mud intrusion across the shaft seal. Proper compensation is also important because of short duration mud pressure increases caused by the telemetry modulation of mud pressure.
There continues to be a need for improved pressure compensation and shaft sealing for MWD instruments.